Embodiments of the invention relate generally to magnetoresistive random access memory (MRAM) and, more specifically to thermally-assisted spin torque transfer MRAM having tunnel junctions with a multi-layer free layer.
Magnetoresistive random access memory (MRAM) is a non-volatile computer memory (NVRAM) technology. Unlike conventional RAM chip technologies, MRAM data is not stored as electric charge or current flows, but by magnetic storage elements. The elements are formed from two ferromagnetic plates, each of which can hold a magnetization direction, separated by a thin insulating layer. One of the two plates is a reference magnet set to a particular polarity; the other plate's magnetization direction can be changed to be parallel or anti parallel to the magnetization of the reference magnet by field or current and is termed as “free magnet” or “free-layer”. The free magnet may also be referred to as a bit, and it may store a “1” or a “0” value. This configuration is known as a magnetic tunnel junction and is the simplest structure for a MRAM bit. A memory device is built from a grid of such “cells.”
One type of MRAM, called thermal MRAM, is configured to have heat applied to the tunnel junction when writing to a bit of the tunnel junction. In particular, the free magnet tends to be stable at a normal operating temperature, and it is more difficult to change magnetic polarity of the free magnet at a normal operating temperature. Providing heat to the free magnet may facilitate changing of a polarity of the free magnet to program a magnetic state of the free magnet. However, a large amount of current necessary to heat the tunnel junction to write to the tunnel junction may make the transistors in an MRAM cell too large for some applications which may contribute to electromigration problems at advanced nodes.